The following changes have
been made for calculating monthly and daily teleconnection indices.

MONTHLY:

The calculation procedure, data source, analysis level,
and base period, have changed for the monthly Northern Hemisphere
extratropical teleconnection patterns and indices shown in Table E1 and Fig.
E7. This has been done to take full advantage of the NCEP/ NCAR Reanalysis,
and to eliminate inconsistencies inherent in the previous data set.

The Northern Hemisphere extratropical teleconnection
patterns are now calculated from the CDAS monthly 500-hPa standardized
height anomalies obtained from the NCEP/NCAR Reanalysis in the analysis region 20°N-90°N.Previously, they were calculated from the monthly 700-hPa
standardized height anomalies, which were obtained from a variety of
analysis procedures prior to 1998, and from the CDAS after 1998. The
anomalies and teleconnection patterns are calculated with respect to the
1950-2000 base period, which is 21 years longer than the previous base
period 1964-1993.

The Rotated Principal Component Analysis (RPCA) remains the basis for
calculating the teleconnection patterns and indices, with the analysis level
now being 500-hPa instead of 700-hPa. In this analysis the ten
leading unrotated EOF’s are first determined for each of the twelve
calendar months, and are based on the standardized monthly anomalies in the
three-month period centered on that calendar month [i.e., The July patterns
are calculated based on the June through August monthly anomalies]. The ten
leading rotated modes are then determined by a Varimax rotation of the ten
un-rotated EOF’s. The teleconnection patterns are a subset of these
rotated modes.

Previously, the indices of all ten rotated modes were calculated
simultaneously for each month in the record using the Least Squares
regression approach outlined by Mo and Livezey (1986, Mon. Wea. Rev., p. 2488-2515).
However,
an examination of all twelve sets of leading rotated modes reveals
ten dominant teleconnection patterns, of which eight to nine appear in each
of the twelve calendar months. Therefore, one or two of the leading modes in
each calendar month are spurious, with no apparent physical meaning.

These spurious modes are now omitted from the Least Squares equations,
whereas before they were retained. The resulting teleconnection indices are
the solution to the Least Squares system of equations, such that they
represent the combination of teleconnection patterns (instead of the
combination of the ten leading rotated modes), which
explains the most spatial variance of the observed standardized height
anomaly field in a given month.

Once the teleconnection indices for all months are obtained, they are
normalized for each teleconnection pattern and calendar month independently
based on their 1950-2000 monthly mean and standard deviation. The resulting
standardized indices are then assembled into a continuous time series
spanning the period 1950-present. The most recent part of these time series
is shown in Table 1 and Fig. E7.

DAILY

The calculation procedure and base period have changed
for calculating the daily NAO and PNA teleconnection indices shown in Fig.
a2.1. These changes have been made to eliminate inconsistencies in the way
that the monthly and daily indices are calculated.

A major change is that the monthly teleconnection patterns are now linearly
interpolated to the day in question, and therefore account for the
seasonality inherent in the teleconnection patterns. Previously, only annual
mean teleconnection patterns calculated from monthly anomalies were used.

A second major difference is that the daily teleconnection indices are now
calculated using the Least Squares regression approach identical to that
used for the monthly indices. Therefore, all of the teleconnection patterns
valid for the day in question are now recognized when calculating the
indices. The daily indices now represent the combination of
teleconnection patterns that accounts for the most spatial variance of the
observed anomaly map on any given day. Previously, the indices represented
the spatial correlation between the annual mean loading pattern of the NAO
or PNA and the daily height anomalies, and did not account for the spatial
overlap that exists amongst the various teleconnection patterns.

A third major difference is that the new teleconnection indices are
calculated from standardized daily 500-hPa height anomalies, as opposed to
non-standardized anomalies. The standardized anomalies are now calculated
based on the 1950-2000 climatological daily mean and standard deviation,
whereas the anomalies were previously calculated from the 1971-2000 base
period daily means.

Key features and changes to monthly teleconnection indices shown
in Table E1 and Fig. E7.

NewOld

Data Source: CDASBlend of CDDB data

and new CDAS

Climatology:1950-20001964-1993

Calculation Level:500-hPa700-hPa

Analysis TechniqueRPCARPCA

(Rotated Principal Component Analysis)

Analysis Period

For determining1950-20001964-1993

Teleconnection patterns

Data InputStandardized monthly Standardized monthly

height
Anomaliesheight anomalies

Teleconnection

RPCA and Least Square
RPCA and Least Squares

Amplitudes

regression using leadingRegression using ten

teleconnectionpatternsleading rotated modes

valid for the month of interestvalid for the month of

interest

Key features and changes to daily NAO and PNA
teleconnection indices shown in Fig. a2.1.